1. Field of the Invention
The present invention relates generally to liquid composite moulding (LCM) processes in which a thermosetting resin is injected into a mold that contains a fibrous body. The resin infuses throughout the fibrous body and is cured within the mold to form a final composite structure. More particularly, the present invention is directed to prolonging the processing window of LCM resins so that the resins can be infused into larger and more complex structures.
2. Description of Related Art
Composite materials are used in a wide variety of endeavors where a structure must have a high strength to weight ratio. The two basic components of composite materials are a resin matrix and a fibrous support structure that reinforces the resin. There are a wide variety of processes for combining resins and fibrous supports together to form structures that can be cured to form final composite structures. The particular process that is used depends on many factors including resin and fiber types, size of the structure, complexity, cost, weight/strength requirements and quality control issues.
For example, in many instances an adequate process for making composite structures is simply to manually impregnate the resin into the fibrous support using a brush or roller. In other situations, the resin is impregnated into the fibrous support using specialized equipment to form a pre-impregnated fibrous support (prepreg). One or more layers of prepreg are then placed on or in a mold to form the desired shape, which is then cured to form the final composite structure.
Liquid composite moulding (LCM) processes are popular processes that have been used widely for making composite structures. Two examples of LCM processes are resin transfer moulding (RTM) and liquid resin infusion (LRI). RTM and LRI are used widely in industries, such as aerospace, where it is necessary to make complex parts or structures that must reliably and reproducibly meet strict limits with respect to strength and weight. RTM processes generally involve injection of the resin into a mould that contains a pre-formed fibrous support. The amount of resin that is injected into the mould and the process conditions within the mould are carefully controlled to insure that the resin is infused completely and uniformly throughout the fibrous support. LRI processes use a mould that has a flexible membrane as the upper tooling surface. The liquid resin is drawn into the mould by applying a vacuum to the mould. The resin passes rapidly over the surface of the fibre preform by means of a layer of highly porous “flow media”. The resin is infused through the thickness of the preform and then cured before demoulding of the finished fibre-resin assembly.
The resins that are used in LCM processes are commonly referred to as LCM resins. Bismaleimide resins and epoxy resins are both popular LCM resins. A particular type of epoxy LCM resin that contains a particulate amine hardener is useful in situations where high toughness is desired. This type of LCM resin is available commercially from Hexcel Corporation (Dublin, Calif.) and Cytec Corporation (Anaheim, Calif.) under the trade names ST15 and PR520, respectively. These epoxy LCM resins contain an epoxy resin component and an amine hardener. The amine hardener is present as particles, which are dispersed throughout the epoxy resin component. In practice, it has been found that the hardener particles are larger than the openings in many fibrous supports. As a result, the particles are inherently filtered out of the LCM resin by the fibrous support when the resin is injected into the mould.
An important consideration for any LCM process that uses particulate hardeners is to make sure that the hardener particles are dissolved before they can be filtered out by the fibrous support. The amine hardener particles that are typically used in amine-hardened epoxy LCM resins dissolve in the epoxy resin component at temperatures on the order of 150° C. to 160° C. This particle dissolution temperature is only 30° C. to 40° C. below the moulding or curing temperature that is normally used for these resins. Accordingly, LCM processors continually face the problem of heating the resin enough to insure particle dissolution, while at the same time avoiding premature gelling or curing of the LCM resin.
The conventional procedure for processing epoxy LCM resins that contain amine particle hardeners is to use a combination of pre-injection heating and mould heating to insure that the hardener particles are dissolved before they can be trapped by the fibrous support. Typically, the mould temperature is kept above the dissolution temperature and the resin is pre-heated to the dissolution temperature prior to injection for only the time necessary to insure that the combination of pre-heating and mould temperature results in dissolution of the particles in a timely fashion. The temperature of the mould is raised to the final curing temperature only after the fibrous structure is completely saturated. This procedure has worked well and insures that the amine hardener is uniformly distributed along with the resin.
The viscosity of the LCM resin, when it is injected into the mould, must be sufficiently low to allow the resin to permeate the fibrous support. The viscosity of the resin does increase gradually over time due to chemical reaction until it reaches a point where the resin can no longer flow and further infusion of the fibrous support is halted. The “processing window” is the time it takes for the resin to go from relatively low viscosity material that flows through the fibrous support to a relatively high viscosity material, which does not flow and makes further resin infusion difficult, if not impossible.
The processing window for LCM resins, which include a particulate amine hardener, in the conventional process described above is on the order of 30 to 45 minutes. This length of processing window works well for many situations where the fibrous structure is sufficiently small and the fibrous support is sufficiently porous to allow complete resin infusion before the resin becomes too viscous. However, there are a number of situations where it can take well over 45 minutes to completely infuse the LCM resin into the fibrous preform. This is especially the case for large and/or complex structures. Accordingly, it is desirable to prolong the LCM processing window for LCM resins that contain particulate hardeners, so that a wider range of composite structures can be moulded.